I agree with Rob that the issue is with QuickTime not processing the frames fast enough and that your file is probably fine. This could be your CPU if you have used a complex algorithm, or your hard drive if it is a large file size. QuickTime itself is often the culprit – try a more efficient player, such as PotPlayer or VLC, and compare results. I very much doubt that the problem lies with your graphics card.
Post the render settings you used in Vegas as well as your CPU, hard drive, RAM and OS specs. This will give anyone trying to assist you a better idea of the possible reasons for the dropped frames.
